Children who develop autism may do so because they have problems processing the toxic metal mercury, researchers have suggested.

US researchers looked at mercury levels in the baby hair of children who later developed autism, a developmental disability that affects how a person communicates and interacts with other people.

They were found to have far lower levels of mercury than children who did not have the condition, according to New Scientist magazine.

The researchers say this could be because autistic children's bodies cannot make use of metals such as mercury properly, or because they have trouble excreting the metal from their body.

The findings of this study are intriguing

National Autism Society spokeswoman

Mercury is suspected by some to be a cause of autism.

A group of parents in the US and Canada are suing health authorities because they believe thimerosal, a mercury-based preservative used in vaccines, could have caused their children's autism.

The MMR vaccine, which some parents fear is linked to autism, does not contain thimerosal.

Baby hair

Experts are divided over whether there could be a link.

Some say more studies need to be carried out before a link between mercury and autism can be confirmed.

But others, such as Louisiana-based Dr Amy Holmes, who carried out this latest research, believe there is a causal link.

Dr Holmes obtained baby hair cuttings taken which had been taken when children were around 18 months old.

She analysed mercury levels in cuttings from 94 autistic children and 45 other children

The average level of mercury in baby hair of children later diagnosed as autistic was 0.47 parts per million, compared to 3.63 per million in the other children.

The more severe the children's autism, the lower the mercury levels found.

Deficiencies

Most of the mercury came from the children's mothers in the form of fillings, injections containing the thimerosal, or through eating a lot of fish.

In the group of non-autistic children, mercury levels rose in line with their mother's exposure.

But levels in the baby hair of the autistic children were low even when their mother's exposure was high.

The researchers say one explanation could be that autistic children's bodies are unable to make use of metals properly, so they could also be deficient in metals which are needed for brain development such as zinc, iron and copper.

Alternatively, they suggest some children might have problems excreting mercury.

Most of the metal is excreted through urine and faeces.

However the researchers suggest that the lack of mercury in the children's hair could be due to the metal being retained in cells, rather than getting into the blood.

'Logical leap'

Some experts say the link is plausible.

Professor Simon Baron Cohen of Cambridge University, said: "This kind of gene-environment interaction is not incompatible with the known heritability [genetic factors] of autism.

"If these results hold up, metal studies on the brain could be revealing."

But Dr Emmanuel Dicicco-Bloom, a child neurologist at the Robert Wood Johnson Medical School in New York warn more evidence is needed before a link between mercury processing and autism can be confirmed.

He told New Scientist it was a big logical leap, not justified by the evidence.

A spokeswoman for the UK's National Autistic Society told BBC News Online: "The findings of this study are intriguing and we would encourage further investigation into what factors might be causing this complex disorder."

The research is due to be published in the International Journal of Toxicology in September.